This invention relates to lifting apparatuses for lifting heavy materials or loads and more particularly to an upper limit position setting device which operates to prevent the lifting of a load higher than a predetermined height in the lifting apparatus.
A conventional lifting apparatus, as is indicated in FIG. 1, comprises: a support 1; a driving section 2 which is provided on the support 1 for driving the apparatus; arms 3 and 4 connected to the driving section 2 through an arm connecting member 5 which is adapted to connect arms 3 and 4 to each other; an operating section 6 which is provided at the end of the arm 4; and a load carrying or supporting member 9 which is adapted to support a load 8 which is to be moved to a desired place. The operating section 6 has a control lever 7 which is operated by an operator 10.
An electrical arrangement of this conventional lifting apparatus is shown in FIG. 2, and a structure of the operating section 6 is illustrated in FIG. 3.
The electrical arrangement shown in FIG. 2 includes an adder 11 which carries out addition and subtraction operations of a target speed signal Vr, an automatic balance signal S and a speed feedback signal Vc to apply a driving signal Ve to an amplifier 12. An electric motor 13 is driven with the aid of a driving signal Vm obtained by amplifying the driving signal Ve by the amplifier 12. A tachogenerator 14 is connected to the rotary shaft of the motor 13 to produce the speed feedback signal Vc proportional to the rate of rotation of the rotary shaft and is connected to a reduction gear mechanism 15. An automatic balance signal generating device 17 is connected to an output shaft 16 of the gear mechanism 15 and when the vertical movement of the load is stopped, produces the automatic balance signal S which is employed to maintain the load wherever it is stopped.
The operating section 6, as is shown in FIG. 3, comprises a potentiometer 20 for providing the target speed signal Vr corresponding to the vertical displacement (angle .theta. of rotation) of the control lever 7 from the neutral position n of the control lever 7, and a pair of springs 21 and 22 to bias the control lever to the neutral position n. One end of the upper spring 21 is connected to the control lever 7, while the other end is connected to the upper wall of the operating section 6. Similarly, one end of the lower spring 22 is connected to the control lever, while the other end is connected to the lower wall of the operating section 6.
With the lifting apparatus thus organized, the operator 10 hangs a heavy load 8 on the load carrying member 9 and moves the load vertically to a desired place by operating the control lever 7.
When the control lever 7 is not operated, the control lever 7 is set at the neutral position n by the mutual action of the upper and lower springs 21, and 22 and accordingly, no target speed signal Vr for driving the motor 13 is produced by the potentiometer 20, that is, the target speed is zero. In this case, an automatic balance signal S corresponding to the weight of the load 8 is produced by the automatic balance signal generating device 17, whereby the position of the load 8 is maintained unchanged, that is, the load is not moved downward. In other words, the driving power of the motor 13 is controlled by the automatic balance signal S which is dependent on the weight of the load 8 so that the load is not moved downward by the weight thereof.
When the end of the control lever 7 is depressed downward for instance, the control lever 7 is turned around the axis of rotation of the potentiometer 20. As a result, the target speed signal Vr is produced by the potentiometer 20 to drive the motor 13 provided in the driving section 2 (FIG. 1). The rotation of the motor thus driven is transmitted through the gear mechanism 15 to the output shaft 16, whereby the arms 3 and 4 are moved. In this operation, the target speed signal Vr operates to drive the motor 13 so that the operating section 6 is moved downward, that is, the load 8 is moved downward.
The downward movement speed, in this case, is proportional to the rotational angle .theta. of the control lever 7. Accordingly, if the control lever is operated to decrease the rotational angle .theta. as the load 8 is moved downward, the downward movement speed of the load is decreased with a decrease of the angle .theta.. Finally, when the angle .theta. becomes zero, that is, the control lever is at the neutral position n, the downward movement of the load is stopped, whereupon the load 8 is automatically balanced by the automatic balance signal generating device 17.
The automatic balance signal generating device 17 detects movement of the shaft 16 caused by the weight of the load, and converts the movement thus detected into, for instance, a voltage variation which is fed back, as the automatic balance signal S, to the amplifier 12 (adder 11). In this operation, since the device 17 is so designed that the automatic balance signal S is obtained in a known manner, e.g. from a potentiometer, which is provided in the device 17 and is frictionally connected to the output shaft 16, in such a manner that the balance signal S is increased and decreased by the relative weight of the load 8, a balance signal S corresponding to the variation of weight of the load is produced and the motor 13 is driven through the amplifier, thereby achieving the automatic balance operation.
In the case when the operator 10 moves the load upward or downward by operating the control lever 7, it is unnecessary to produce the automatic balance signal S, since movement is controlled substantially by target speed signal Vr. However, it is necessary that the automatic balance signals S be produced with respect to all of the at rest positions of the operating section 6 ranging from the uppermost position to the lowermost position.
On the other hand, when the control lever 7 is moved upward, a target speed signal Vr corresponding to the angle .theta. is produced to move the load 8 upward. Similarly as in the case when the control lever 7 was moved downward, the upward movement of the load is stopped by operating the control lever in such a manner that the angle .theta. becomes zero, that is, the automatic balance of the load is obtained.
When the operator 10 removes his hand from the control lever 7, the control lever 7 is set at the neutral position n by the mutual action of the springs 21 and 22 and the lifting apparatus is under the automatic balance condition.
As is clear from the description above, the conventional lifting apparatus forms a feedback control system, and the load is moved upward or downward in accordance with movement of the operator's hand holding the control lever.
Furthermore, in consideration of simplification in construction, convenience in installation and distance in load movement, the lifting apparatus is in the form of a crane having an arm. Therefore, the lifting arm is less mechanical rigid than machine tools. However, in general, the lifting apparatus has a mechanically determined upper limit position (hereinafter referred to as "mechanical upper limit position" when applicable) to which it can lift a load. Accordingly, if the load is lifted above the mechanical upper limit position, a relatively too great a mechanical stress is applied to the power transmission mechanism of the lifting apparatus. This is undesirable with respect to safety of operation. Furthermore, if the operator removes his hand from the control lever when the load has reached the mechanical upper limit position, the movement of the load is stopped with an excessively great stress being imparted to the power transmission mechanisms. This is dangerous.
In order to overcome these difficulties accompanying the conventional lifting apparatus, the following method has been employed. That is, the arrival of the load to the upper limit position is detected, for instance, by a limit switch thereby to stop the motor, and furthermore the load is moved downward by operating a push button switch, which produces a command signal for moving a load downward, instead of by operating the control lever. Thus, it is impossible to smoothly move a load upward or downward by the conventional lifting apparatus. Accordingly, it has been strongly demanded in the art to improve the above-described control system of the lifting apparatus.